The production of moldings having a compact surface by the isocyanate polyaddition process is known in principle. It may be carried out, for example, by introducing a reactive and, if desired, also foamable mixture based on compounds having several reactive hydrogen atoms and polyisocyanates into a mold (see, for example, German Auslegeschrift No. 1,196,864). The compounds having reactive hydrogen atoms used are mainly polyethers and polyesters having hydroxyl groups. Examples of suitable polyisocyanates include tolylene-2,4- and -2,6-diisocyanates and isomeric mixtures thereof, as well as polyphenylpolymethylene polyisocyanates obtained by a process of anilineformaldehyde condensation followed by phosgenation. Water and/or fluorinated hydrocarbons, for example, may be used as blowing agents. The catalysts known for the production of polyurethanes are generally also used.
With suitable choice of the starting components (other substances, e.g. chain lengthening agents, such as glycols or diamines, may also be used) it is possible by this process to obtain both elastic products and rigid products, as well as variations between these extremes.
Systems which contain diamines as chain lengthening agents cannot generally be processed by a one-shot process. In these cases, a prepolymer containing isocyanate groups must first be prepared. This prepolymer is then reacted with the diamine in a second step to yield the high molecular weight elastomer (German Auslegeschrift No. 1,240,654).
For molded products which will be subjected to severe stresses in use, only slightly branched starting materials which give rise to products with elastomer-like properties are generally used. Moldings of this type have long been in production on a commercial scale (e.g. as shoe soles for the shoe industry). Large moldings have lately come into use in the automotive industry.
Processing of the raw materials to produce automotive body parts is primarily carried out by the so-called "reaction injection molding process" (RIM). This process involves a technique of filling the mold by which highly reactive, liquid starting components are injected into the mold within a very short time by means of high output, high pressure dosing apparatus after they have been mixed in so-called "positively controlled mixing heads."
The RIM process is widely known and a detailed description of the technology thereof may be found, for example, in the following references:
______________________________________ Piechota/Rohr: "Integralschaumstoffe" Carl Hanser-Verlag, Munich/Vienna, 1975; Prepelka/Wharton: "Reaction Injection Molding in the Automotive Industry", Journal of Cell. Plastics, Vol. II. No. 2, 1975; Knipp: "Plastics for Automobile Safety Bumpers", Journal of Cell. Plastics, No. 2, 1973. ______________________________________
The reaction injection molding technique may be used for producing large moldings weighing from 3 to 10 kg or more, such as the flexible car body parts also known in the motor industry as "soft face elements", i.e. reversibly shaped front and rear parts of motor vehicles.
The following technical advance is generally achieved by the RIM procedure: large quantities of two liquid, highly reactive starting materials are delivered mechnically within a very short time (from about 2 to 4 seconds) and mixed at the same time and introduced into a mold in which the mixture is cured to yield the finished product within a time (from 1 to 2 minutes) which is also very short for polyurethane materials.
Realization of this new technology required a solution to the following three problems:
1. In view of the high reactivity of the two starting components (polyisocyanates and compounds which are reactive with polyisocyanates), the reaction mixture must be introduced into the mold within the shortest possible time which should not exceed the cream time (i.e., the time between mixing of the reactants and the first visible signs of a chemical reaction). This necessitated the development of highly efficient axial and radial piston pumps which when installed in high pressure machines were capable of delivering at a rate of from 2.5 to 6.5 kg/second. Machines of this type have been described, for example, in German Offenlegungsschriften No. 1,778,060 and No. 2,146,054.
2. Production of a faultless molding required not only exact dosing of the two components to keep them at a particular ratio prescribed by the given formulation over the whole period of injection, but also required intimate mixing of the components from the first to last drop. Perfect mixing is made enormously difficult by the fact that due to their high flow velocities, the two components have only a very short residence time in the mixing chamber of the mixing head. This problem could be solved by using so-called "positively controlled mixing heads" which operate on the principle of "counterflow injection" (see e.g., U.S. Pat. No. 3,709,640 and No. 3,857,550, and German Offenlegungsschriften No. 2,007,935 and No. 2,364,501).
3. When the reaction mixture enters the closed mold, it almost instantly displaces the air contained in it. To ensure that this does not lead to inclusions of air in the reaction mixture and hence faults in the end product, the liquid streaming into the mold must, in effect, "push" the air forward in front of it in the form of a "flow front" and expel it through predetermined slots. To ensure complete absence of turbulence during filling of the mold, the material must enter the mold over a wide front along the wall of the mold in a laminar stream. This has been achieved by developing a certain technique of injection through so-called "film gates" described in German Offenlegungsschriften No. 2,348,658 and No. 2,348,608.
U.S. Pat. No. 3,655,597 broadly discloses reacting polyisocyanate, polyol, blowing agent, diamine, and catalyst. The reference suggests the use of polymethylene diisocyanates and aromatic diisocyanates such as tolylene diisocyanate as possible isocyanates. The reference suggests aromatic diamines containing no negative group in the ortho-position to the amino group. Phenylene diamines and tolylene diamines are mentioned in the list of possible diamines fitting this description. The reference specifically discloses 3,3'-dichloro- 4,4'-diaminodiphenylmethane (MOCA) and a mixture of MOCA and o-tolylenediamine (TDA). However, the use of TDI and MOCA is too slow for practical use in a RIM system and the use of TDA as a chain extender causes the mixture to react so fast that it sets before it can be poured into a mold. Furthermore, TDA has only a limited compatibility with polyether polyols so that it can only be used in relatively small quantities. Even if small quantities are used, care must be taken that the chain extender does not crystallize.
Abandoned U.S. Pat. application No. 672,694 which was incorporated by reference in column 2, line 18, of U.S. Pat. No. 3,655,597 discloses tolylenediamine (TDA) in combination with polymethylene polyphenylisocyanate in the Examples. As mentioned, the mixture containing TDA reacts too fast to be poured into a mold. TDA also has a very limited compatibility with polyether polyols so that it can only be used in relatively small quantities. Even with small quantities care must be taken so that the TDA does not crystallize.
Defensive Publication No. T 919,009-Strassel et al suggests a process of reacting polyisocyanate, polyol, poly- amines or amino alcohols, foaming agent, and catalyst. TDI and MOCA are disclosed. As noted above, the use of TDI and MOCA is too slow for practical use in a RIM system. The polyisocyanates suggested in the Strassel reference include (page 5, lines 23 and 24) toluene diisocyanate, diphenylmethane diisocyanate, polymethylenepolyphenyl isocyanate, and mixtures thereof. TDI and MDI are used alone and as a prepolymer blend. The reference discloses MOCA, dichlorobenzidine, and a mix of MOCA and metaphenylene diamine as the chain extender. On page 6, lines 13 and 14, of the application of the Defensive Publication, dichlorobenzidine is described in the same class as MOCA. The use of MOCA results in too slow a setting time for practical use in a RIM system.
U.S. Pat. No. 3,586,649 suggests foam mixtures comprising polyisocyanate, polyol, aryl diamines, blowing agent and catalyst. Column 12, line 24, lists diphenylmethane-4,4'-diisocyanate in its long list of possible isocyanates. TDI and MDI are disclosed as possible isocyanates. Four classes of aryl diamines are suggested. The four classes are represented by general formulas, none of which read on the special diamines of the instant invention. MOCA is disclosed as the chain extender.
U.S. Pat. No. 3,752,790 is directed to chlorinated toluene diamines as curing agents. In column 7, Tables I and II, the reference indicates that chlorinated toluene diamines have longer demolding times than generally used curing agents. The Tables record times of 15 to 20 minutes and 25 to 30 minutes. MDI and TDI are disclosed as possible isocyanates to use with the chlorinated toluene diamines.
U.S. Pat. No. 4,002,584 is directed to halogenated diamino diphenylmethanes as chain extenders. MDI and TDI are disclosed as possible isocyanates.
U.S. Pat. No. 3,583,926 discloses the use of sterically hindered aromatic polyamines and a MOCA-type diamine. U.S. Pat. No. 3,591,532 also discloses MOCA-type diamines.